1. Field of the Invention
The present invention relates to an insulation detecting device, and especially relates to an insulation detecting device suitable for insulation detection of an ungrounded direct-current power supply mounted on a vehicle that uses driving force by electricity.
2. Description of the Related Art
In recent electric vehicles or hybrid cars, a battery assembly as a high-power, high-output, and compact direct-current power supply (hereinafter, abbreviated as direct-current power supply) is mounted. The direct-current power supply is configured such that a plurality of battery cells having a positive electrode and a negative electrode is connected in series, and an output voltage is 200 V (volt) or more. Therefore, positive and negative power supply lines that are outputs of the direct-current power supply are configured to be electrically insulated from a vehicle (vehicle body), that is, the direct-current power supply is configured to be ungrounded. The vehicle (vehicle body) is configured not to be used as the ground of the direct-current power supply.
The vehicle having such a configuration is typically configured to monitor an output voltage of the direct-current power supply and to monitor an insulation state between the direct-current power supply and the vehicle (vehicle body) using an insulation measurement technology in a flying capacitor system, like an insulation detecting device described in Japanese Patent Application Laid-open No. 2004-170103, for example. The insulation detection device described in Japanese Patent Application Laid-open No. 2004-170103 is configured from a flying capacitor in a state of flying from a ground potential, that is, the ground, a plurality of resistances and diodes, and a plurality of switches that controls connection between the direct-current power supply, and the flying capacitor and the plurality of resistances and diodes.
In the insulation detecting device, the switches are appropriately controlled, and the flying capacitor is charged in a state where one terminal of the flying capacitor is connected to a positive electrode of the direct-current power supply and the other terminal of the flying capacitor is connected to a negative electrode of the direct-current power supply, and a voltage (voltage corresponding to the output voltage) charged in the flying capacitor is measured. Following that, the switches are appropriately controlled, and the voltage charged in the flying capacitor is discharged. Next, the switches are appropriately controlled, and the flying capacitor is charged with the direct-current power supply in a state where the one terminal of the flying capacitor is grounded through a ground fault resistance, and a first voltage charged in the flying capacitor is measured, as measurement of a ground fault resistance measured voltage. Next, after the first voltage is discharged, the switches are appropriately controlled, and the flying capacitor is charged with the direct-current power supply, and a second voltage charged in the flying capacitor is measured in a state where the other terminal of the flying capacitor is grounded through the ground fault resistance. When the measurement is completed, the second voltage is discharged. Then, the insulation state, that is, existence or non-existence of ground fault of the direct-current power supply (including the power supply line connected to the direct-current power supply) is calculated based on the measured output voltage, first voltage, and second voltage of the direct-current power supply.
In the insulation detecting device described in Japanese Patent Application Laid-open No. 2004-170103, at the time of measurement of the first voltage and the second voltage, only one of terminals of the flying capacitor is connected to the power supply line of the direct-current power supply, and the flying capacitor is charged. Meanwhile, in the power supply line of the direct-current power supply, a capacitance called Y capacitor, that is, a capacitance associated with arrangement of a capacitor (noise removal capacitor) for removing high-frequency noises and the like, and a floating capacitance formed between the power supply line and the ground are formed. Therefore, in the insulation detecting device described in Japanese Patent Application Laid-open No. 2004-170103, to suppress an influence of an electric charge from the Y capacitor on the flying capacitor when the flying capacitor is charged with the direct-current power supply in a state where one or the other terminal of the flying capacitor is grounded through the ground fault resistance, a flying capacitor having a sufficiently larger capacitance than the Y capacitor needs to be used. However, to measure the output voltage of the direct-current power supply with high accuracy, the capacitor needs to be charged to a state of full charge or nearly full charge, and there is a problem that, when the flying capacitor having a large capacitance is used, an extremely long charge time is required.
Further, measurement in a short time rather than the measurement accuracy, like measurement in an insulation state immediately after a power supply of the vehicle is turned ON, or highly accurate measurement of the output voltage of the direct-current power supply in a short time are desired.